1. Field of the Invention
The present invention relates to a novel polymerization apparatus improved in heat dissipation ability.
2. Description of the Prior Art
Conventionally, as polymerization apparatuses used in suspension polymerization or the like of a vinyl chloride based-monomer, for example, polymerization apparatuses are known that comprise a polymerization vessel equipped with a reflux condenser, a cooling jacket, an agitator, a nozzle through which raw materials are charged, a nozzle through which the polymer is withdrawn, and the like. In these polymerization apparatuses, the removal of heat (dissipation of heat) of polymerization is effected mainly through said reflux condenser and jacket.
Further, as polymerization apparatuses that adopt other heat dissipation system, there are also known a polymerization apparatus comprising a polymerization vessel with a cooling coil, a draft tube, and the like attached thereto and a polymerization apparatus having a cooling apparatus outside thereof wherein the polymerization reaction product is passed through the outside cooling apparatus where the heat of the polymerization reaction product is removed and then the polymerization reaction product is returned to the polymerization vessel.
However, in suspension polymerization particularly of vinyl chloride or a monomer mixture mainly made up of vinyl chloride (hereinafter referred to as a vinyl chloride based-monomer), to place an internal structure, such as a cooling coil and a draft tube, in the polymerization vessel results in an increase in power required for agitation. That also causes the mixing performance of the charged materials to drop thereby allowing part of the flow in the polymerization vessel to slow down. As a result, the temperature in the polymerization vessel becomes nonuniform, so that the sizes of grains of the obtained polymer become nonuniform and further polymer scale is liable to be deposited on the inner wall of the polymerization vessel. The scale causes fish eyes to increase, which damages the quality of molded items considerably. Thus, to remove heat by such a means makes the structure in the polymerization vessel complicate and therefore leads to various problems.
Further, the method wherein a cooling apparatus is placed outside a polymerization vessel and a polymer mixture is circulated through that cooling apparatus requires, in addition to the management and maintenance of the polymerization vessel, the management and maintenance in the circulation line and makes such an operation as washing with water apt to be complicate. Further, the use of this outside circulation apparatus is attended with the problem that the quality of the obtainable polymer is inferior to the case wherein such an outside circulation apparatus is not used.
By the way, if the size of a polymerization vessel itself is made large in order to improve the productivity or if a polymerization method that can shorten the polymerization reaction time is adopted, the amount of generated heat per unit time increases and therefore it is required to increase the heat dissipation efficiency. For that purpose, it is possible to use a refrigeration machine to lower the temperature of cooling water to be flowed through a cooling jacket, but this is not preferable from an economical point of view.
Further, in the case of a large-sized polymerization vessel having a volume of 40 m.sup.3 or more, since the increase in heat dissipation by a jacket has a limit itself, it becomes required to increase the amount of heat removed by a condenser. However, to increase the load of heat dissipation on the reflux condenser brings about an increase of foam of the slurry in the polymerization vessel to cause the slurry to overflow into the reflux condenser, and as a result the ability of the reflux condenser of removing heat is dropped or scale of the polymer happens to be deposited on the interior of the reflux condenser. In some cases, the pipeline is clogged with the overflowed polymer particles, making the operation of the reflux condenser not operable.
Further, the use of a reflux condenser from the initial stage of polymerization results in such problems that the particles of the obtained polymer are made coarse and that a frothy polymer is produced. Therefore, in the case wherein a large-sized polymerization vessel is used or the polymerization reaction time is shortened, the problem cannot be avoided that the heat dissipation only by a reflux condenser and a jacket becomes unsatisfactory. Therefore, the heat dissipation by the jacket and the reflux condenser has to be supplemented with other cooling system.
However, where, as a means of supplementing this insufficient heat dissipation, a cooling coil, a draft tube, an outside cooling apparatus, or the like is used to increase the heat transfer surface area for cooling, there is a problem that the quality of the obtained polymer is lowered or scale is formed.